1. Field of the Invention
The present invention relates to a color expressing method, a color image reading apparatus, and a color image processing apparatus.
2. Related Background Art
For example, a conventional color expressing method in a color image reading apparatus adopts a color system of R, G, and B signals determined by, e.g., an NTSC (National Television Sub-Committee) scheme and represented by R, G, and B in FIG. 6 as spectral sensitivity of a tricolor separation system. The color system is determined in accordance with the color emission characteristics of R, G, and B phosphor materials of a cathode-ray tube as coloring materials used in a television receiver. A method using the spectral sensitivity in the XYZ colorimetric system of the CIE (Commission Internationale del'Eclairage) or a tricolor separation system having a narrow-band (narrow band width) spectral sensitivity for measuring the Y (yellow), M (magenta), and C (cyan) densities of inks or colorants used in an original (transmission or reflection) is known.
In the prior art described above, since marks representing R, G, and B chromaticity values or color degrees are plotted within a spectral locus, as shown in FIG. 6, the spectral sensitivity characteristics of the tricolor separation system used in a color image reading apparatus for generating R, G, and B signals which satisfy these chromaticity values must have a theoretically negative region, as shown in FIG. 7. However, the spectral sensitivity characteristics having a negative region cannot be realized in practice. The spectral sensitivity characteristics are therefore approximated by spectral correction (i.e., the negative region is eliminated or correction is performed as indicated by broken lines), as shown in FIG. 8, or corrected in accordance with linear conversion. However, with this method, the color characteristics of a target original or an object are read with a large amount of errors. Even if the color characteristics are accurately read, colors plotted outside the triangle defined by chromaticity values of color components emitted from the above-described phosphor materials such as colors represented by marks x in FIG. 9 have negative signal values. This phenomenon causes difficulty in processing signals. If each negative signal is set to "0", the corresponding color cannot be expressed, resulting in an incorrect reproduction of the color.
In use of a color expressing method of the XYZ colorimetric system of the CIE, signal values read for expressing colors along the x- and y-axes in the color degree diagram or chromaticity diagram in FIG. 6 are not negative. The spectral sensitivities for realizing the XYZ colorimetric system for realizing the above color expressing method are as shown in FIG. 10 (the signal values are normalized with maximum sensitivity values). As is apparent from FIG. 10, since the full-width at half maximum of the y spectral sensitivity for forming a Y signal is wide, filters having different spectral transmittances must be combined to satisfy this spectral sensitivity. Digital values of the X, Y, and Z signals cover a wide region on the color degree diagram, and nonexisting colors are also assigned with signal values. The number of effective data becomes only about 65% of all data to be quantized (FIG. 14). FIG. 14 shows chromaticity points obtained when X, Y, and Z signals are quantized with six levels. It is apparent that chromaticity points outside the spectral locus are present, thus resulting in inefficient signal value utilization.
In a chromaticity meter using a narrow-band spectral filter used in printing equipment, the color separation characteristics of an object such as a color picture are poor except when the spectral characteristics of inks and the like used in an original are already known.
For example, in a system such as a digital color copying machine integrally including a color image reading apparatus, a color image processing apparatus, and a color image output apparatus, a unique color data expressing method is used for operations from color image inputs to color image outputs in accordance with the spectral sensitivity characteristics of a tricolor separation system and the spectral characteristics of inks and colorants.
In a color image reading apparatus such as a color image reader, read signals are obtained using a color data expressing method depending on the spectral sensitivity characteristics of a tricolor separation system used in this apparatus. Similarly, in a color image output apparatus such as a color printer of an ink-jet or thermal transfer system, colors expressed by input signals vary depending on the types of color printers. That is, a color data expressing method unique to each printer is employed.
As described above, in a color monitor such as a color television receiver, an RGB color data expressing method complying with the NTSC standards shown in FIG. 14, as described above, is employed in Japan. This color data expressing method is determined in accordance with the color characteristics of the CRT R, G, and B phosphor materials serving as coloring materials used in the television receiver.
As in the digital color copying machine described above, however, when the color image reading apparatus, the color image processing apparatus, and the color image output apparatus employ different color expressing methods suitable therefor, in order to cause the color image processing apparatus to process color data read by the color image reading apparatus and cause the color image output apparatus to output the processed color data, conversion operations must be repeated to obtain data suitable for each color expressing method of each apparatus due to differences in spectral sensitivity distributions and spectral characteristics of the apparatuses. This repetition results inconvenience and loss of time.